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FM ALIGNMENT
NECESSARY INSTRUMENTATION
• FM Generator (less than 0.05% THD.)
• Stereo Modulator (less than ;0.05% THD., more than 50dB Sep.)
• Audio Generator (not necessay if FM Generator has built-in sweep, e.g., SOUND TECHNOLOGY ST 1000A and
ST iO2OA)
• AC VTVM's (or one with a LeftRight switch)
• THD Analyzer (resolution less than 0.1%)
• Oscilloscope (5mV or better sensitivity, X input capability)
• Frequncy Counter
• VOM or DMM (high impedance, must read in mV)
• 75 ohm Dummy Antenna
IMPORTANT
1) Before alignments commence, release IF NARROW and MONO switches (out), Switch FM NR off (in).
2) IF FM Generator is not synthesizer-type, be sure to check its frequency with FREQ counter when adjusting detector and
multiplex decoder circuits.
A.
SYNTHESIZER FREQUENCY
1. Connect Frequency Counter between TP-104 and ground.
2. Tune to 98MHz (No RE input needed).
3. Adjust C-201 so that the local oscillator frequency shown by the frequency counter reads 108.700MHz.
TOLERANCE: 108.700MHz +/—2KHz
B.
FRONT-END ALIGNMENTS
Alignment of the front-end should only be necessary after repair to the front-end or crystal oscillator circuits.
a) TUNING VOLTAGE
It is essential to check tuning voltage before aligning the rest of front-end.
1. Connect DMM between TP-105 and ground.
2. Tune to 88MHz, and adjust L-6 if the voltage is incorrect.
TOLERANCE: 3.6V +/—0.5V.
3. Tune to 108MHz and confirm that the voltage is within the following specification.
SPECIFICATION: 21-25V.
b) TRACKING
1. Connect FM Generator (150KHz sweep, 100V output) to 75ohm antenna input and Detector Probe to Pin 1 of
IC 102 with ground to the tunershield.
2. Set tuner to 106MHz, entor into Preset 8, and tune the generator so that curve appears on Oscilloscope.
3. Adjust C-2, C-9, C-li and C-14 for maximum curve height on the oscilloscope while reducing RF input to keep
entire curve on display.
4. Set tuner to 90MHz, enter into Preset 1, and tune the generator so that curve appears on the oscilloscope.
5. Adjust L-1, L-2, L-3 and L-4 for maximum curve height.
6. Repeat above steps 2, 3, 4 and 5 (use Preset 1 and 8) till both frequencies are at maximum curve height.
SUGGESTED INSTRUMENTATION HOOKUP
OSCILLOSCOPE
FM
GENERATOR
AUDIO OSC
1
0
0
I
USE ONL'? IF FM GENERATOR WITH BUILT-IN SWEEP UNAvAILABLE
I
WITH BUILT-IN SWEEP
lE: ST 1000A
DISTORTION METER
DISTORTION
OUTPUT
SCHEMATIC DIAGRAM OF DETECTOR PROBE
Cl
02
330p
00
4.
00
Diodes should be point-contact
FROM
OUT
germanium; Some commonly
TUNER C>'
TO
scoc
available types are: 0A80
0A8 1
AA1 19
1N60
1 N34
1N22
PICTORIAL DIAGRAM OF DETECTOR PROBE
Ri
CENTER LEAD
TOPINI
Cl
02
SCOPE
Keep leads
as shortas possible, to
minimize stray signal pickup.
AT FRONTEND
5
6
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